This invention relates to a low-velocity electron excited phosphor used in a luminous display section of a fluorescent display device, and more particularly to a low-velocity electron excited phosphor produced by ion implantation and a method for producing the same.
A conventional method for producing a phosphor by ion implantation is disclosed in Japanese Patent Publication No. 17624/1978. An ion implantation apparatus used for practicing the method generally includes a vacuum unit in which a substrate having a matrix crystal for a phosphor deposited thereon in a predetermined pattern is received, an ion source section containing an activator, an ion drawing-out section for drawing out the activator ionized, and the like. The ion source section is adapted to produce vapor of the activator and ionize it, resulting in ion of the activator being produced. The ion drawing-out section and the like serve to impart kinetic energy to the ion to impinge it on the substrate, so that the so-formed ion of the activator is implanted in the matrix crystal for the phosphor. After implantation of the ion, the substrate is heated to a temperature of 1000.degree. C. over a period of time as long as 1 hour or more, resulting in being subject to annealing. This leads to thermal diffusion of the activator into the matrix crystal.
The conventional method described above is adapted to treat the substrate on which the matrix crystal for the phosphor is deposited in a display pattern. Unfortunately, the conventional method fails to increase efficiency with which the substrate is treated, because the ion implantation apparatus is constructed so as to receive only one substrate therein for every treatment. Also, the conventional method fails to practice the annealing step at an increased rate of temperature rise. For example, a period of time as long as 1 hour or more is required for rising the temperature to a level of 1000.degree. C. Also, cooling of the substrate thus heated which is carried out after it is kept at 1000.degree. C. for a predetermined period of time requires a period of time as long as 1 hour or more. Thus, it will be noted that the conventional method leads to significant deterioration in operating efficiency.
Further, the fact that the annealing step in the conventional method requires one hour or more for heating the substrate to a level of 1000.degree. C. causes the activator implanted in the matrix crystal to be thermally diffused into the matrix crystal deeply. More particularly, as shown in FIG. 12, a conventional phosphor produced according to the above-described conventional method causes even a concentration of the activator implanted in a portion of the matrix crystal as deep as 1 .mu.m from a surface thereof to be as large as about 30 supposing that a concentration of the activator on the surface of the phosphor is 100. In this respect, it is considered that when lower-velocity electrons accelerated at an acceleration voltage of at most about 100V such as electrons utilized in a fluorescent display device are impinged on the phosphor, only a portion of a phosphor extending to a depth as small as tens angstroms from a surface of the phosphor is excited due to the impingement. Thus, it is desirable that the activator acting to promote or control luminescence of the matrix crystal concentratedly exists at a portion of the matrix crystal extending to a small depth from a surface of the matrix crystal, therefore, the activator implanted or doped in a portion of the phosphor significantly apart from the surface thereof as in the prior art fully fails to contribute to luminescence of the phosphor.
The amount of activator implanted in a matrix crystal in manufacturing of a phosphor is extraordinarily increased as compared with, for example, the amount of P or the like implanted in Si in manufacturing of a semiconductor. Thus, in manufacturing of a phosphor by conventional ion implantation, it is desirable that a dose rate (ion current) is increased to a level as high as, for example, 10 .mu.A, to thereby reduce a period of time required for ion implantation. However, ion implantation of the activator in the matrix crystal in the form of a powder at a high dose rate causes the matrix crystal to generate heat. Unfortunately, the so-generated heat is accumulated in the matrix crystal because it is in the form of a powder exhibiting low thermal conductivity, resulting in a surface of the phosphor being decomposed and/or melted.
In order to avoid such a disadvantage, it is proposed that the dose rate is set to a level as low as 1 .mu.A to prevent the matrix crystal from generating heat. Unfortunately, this causes a period of time required for the ion implantation to be extraordinarily increased. For example, a period of time as long as about 40 hours is required to implant a Mn ion in a predetermined amount in a matrix crystal of ZnGa.sub.2 O.sub.4.